The present invention relates to an apparatus and method for blending dry comminuted, granular or powdered materials.
Cement is an extensively used product in all industrialized areas of the world. It has now becoming common in certain third world countries. Any advances in technology to make cement more accessible to poorer countries is desirable, along with methods to reduce the amount greenhouse gases released during production.
The most cost effective way to make cement affordable to all countries is to use cheaper, more readily available components in the cement mix and/or finding cheaper way to combine the ingredients of cement. At the same time however care must be taken to ensure that the cement maintains it""s physical properties.
The current method of producing cement involves costly rotary kiln firing of a mixture of ground limestone, clay and small amounts of fluxing agents, alumina and iron. The product of this combustion is called Clinker. Clinker is then finely ground and homogeneously mixed with gypsum to produce traditional cement. This process is not only costly due to the large amount of fuel consumed by the firing process, but also environmentally unsound in that a large amount of carbon dioxide (CO2) released into the atmosphere.
Carbon dioxide (CO2) is a greenhouse gas that rises into the upper atmosphere and traps solar heat, therefore contributing to global warming. Combustible reactions using fuel release large amounts of carbon dioxide(CO2) due to incomplete combustion. As well the chemical reaction that produces Clinker also produces carbon dioxide (CO2) due to calcination of the limestone. Therefore cement production is a major contributor of greenhouse gases into the atmosphere, having a total evolution of one ton of carbon dioxide gas (CO2) per one ton of Clinker. In view of the importance of reducing greenhouse gases, new methods of production are required or at least more efficient processes.
One method of creating a more efficient process is to use ingredients that will increase the volume of cement produced, thereby increasing the percent yield of the process. Pozzolanic materials are compounds that have cementitious properties and when mixed with lime, water and gravel produce concrete that has the same or even superior strength than traditional concrete. Addition of pozzolanic material to cement increases the amount of product. In this case fly ash is the pozzolanic material examined.
Fly ash is a by-product of combusted coal. Coal is commonly used to fire steam boilers, steam is then used to generate electrical power. Since coal is not pure carbon, non-combustible material is generated and this is called fly ash. This fly ash consists of fine, minute particles of glassy spheres. These particles are carried out of the boilers by flue gases and collected in gas filters. Chemically, fly ash is predominantly oxides of silicon, aluminum and iron with small percentages of oxides of calcium, magnesium, titanium and the alkalis. Physically, fly ash is a fine powder comprised of minute glassy particles, mainly spherical, with an average particle diameter of 10-15 microns. This is a waste product that is difficult to handle and due to anti-pollution regulations worldwide, a costly matter to dispose of.
Due to the finely divided size and the cementitious nature of this fly ash, blends using the correct percentages of fly ash with cement produce a superior product. This blended product is known as blended cement or pozzolanic blend.
The use of fly ash in concrete provides significant advantages to both the plastic and hardened properties of concrete. In the plastic state, workability is significantly improved and a lower water-cement ratio can be used which improves the concrete strength. In the hardened state long term strength and sulfate and alkali-silica reaction resistance are improved and chloride penetration is reduced.
Fly ash is commonly blended with cement to increase the volume without jeopardizing the integrity of the concrete. Since it is a waste product of another process it is readily available and cheap, compared to the cost of producing Clinker. Using fly ash with cement provides a lucrative and industrial way of utilizing it and keeping it out of the landfills. The fly ash replaces clinker in cement with a 1:1 ratio. Therefore reducing the release of carbon dioxide (CO2) from combustion and calcination by a 1:1 ratio as well. In other words the use of one ton of fly ash replaces one ton of clinker and therefore one ton of carbon dioxide (CO2) released into the atmosphere. When compared to the 100 million tons of cement used in the US in 1997 and the 100 million tons of carbon dioxide (CO2) produced, it is obvious that a 1:1 ratio reduction would make a significant difference. 20 to 25% of fly ash is commonly used in the US with percentages increasing or decreasing depending on the end product. A 20% substitution with fly ash would of reduced the carbon dioxide (CO2) evolution by 20 million tons and kept 20 million tons of fly ash out of the landfill, as well as kept dumping costs down for the respective power companies.
To maintain unified physical properties throughout mixed concrete, the cement must be a homogeneous mixture of all of it""s components. Mixing of fly ash with the limestone of cement has an inherent difficulty due to different physical properties such as density and flowability. The success of the fly ash blended cement requires reliable proportioning and thorough, homogenous blending.
Conventional methods of blending the fly ash with the cement have several difficulties. The percentage of fly ash that can be mixed easily is kept to very low levels. These methods also require a large amount of electrical energy and consequently involve the release of carbon dioxide (CO2) gas produced by electrical power generation. In some cases, because of the nature of the materials, using conventional commercially available dry mechanical mixers can lead to separation instead of the expected blending.
For example, in one method of blending fly ash with cement, the fly ash is added during the grinding and blending of the clinker and gypsum. In this case, all of the materials are placed into a large ball mill. The ball mill is a large metal cylinder that contains a number of metal balls. As the cylinder is rolled, the balls cascade to grind and blend the materials. The ball mill is large, expensive and requires a lot of energy to operate. If the fly ash is added at this stage, the amount of time that the ball mill must be operated must be increased. Furthermore, the end product will only have a fixed percentage of fly ash that cannot be adjusted based on the particular needs of an individual project. Also, because of the different physical properties of fly ash and clinker, one of the components may L)e overground resulting in wasted energy.
In other conventional methods of blending, the fly ash and cement are blended in a separate blender. Conventional blenders are either mechanical, using mechanical ribbons or screws to blend the materials, or pneumatic, in which compressed air is passed through the materials to blend the materials. Both of these methods again use significant quantities of energy due to the weight of the materials and have problems with achieving an appropriate degree of homogeneity.
An energy efficient, inexpensive method of homogeneously blending fly ash with cement would result in a large reduction in the emissions of carbon dioxide (CO2) gas, a reduction in the amount of fly ash put into landfill and the resulting risk of ground water pollution, and would provide for cheaper, more easily accessible concrete, particularly in developing nations where the capital cost of a ball mill or similar blending device is prohibitive. Also, the proportions of the flyash/cement mix could be varied easily to suit the specification of a particular end-user.
Other industrial blending operations in which dry materials must be blended use similar methods and devices to those described above, these industrial process would also benefit from an energy-efficient, inexpensive method of homogeneously blending dry comminuted, granular or powdered materials.
In order to overcome the problems described above, according to an embodiment of the invention, there is provided an improved dry blending apparatus for blending particulate dry comminuted materials including a substantially upright wall defining a blending enclosure, a feed means for feeding the dry comminuted materials into said blending enclosure, and a rotational distribution mechanism for distributing said dry comminuted materials onto said upright wall by causing said dry comminuted materials to fly outwardly by centrifugal force, and includes mechanism for raising and lowering the distribution plate relative to the upright wall.
The use of an energy efficient centrifugal force to distribute the comminuted materials results in an even distribution of comminuted materials on the upright wall. The dry materials collect on the upright wall in a very thin layer over a large collection area. The comminuted materials are partly mixed when they reach the upright wall and then mix further as they slough down off the upright wall due to gravity. Resulting in a very homogeneous mixture of the dry comminuted materials.
The blending enclosure is preferably substantially cylindrical but may also have non-circular cross-section.
In this embodiment of the invention, the dry blending apparatus may also include a tapering chute connected at a base of the substantially upright walls. The tapering chute is preferably substantially conical but may also have a non-circular cross-section. The homogeneous mixture sloughs off the upright collecting walls into the tapering chute.
The rotational distribution mechanism may include a distribution motor, a distribution shaft, driven by the distribution motor, placed along a central axis of the blending enclosure, and a distribution plate connected to the shaft. With this arrangement, the distribution plate and the feed mechanism are positioned such that the dry comminuted materials are fed from the feed mechanism onto the distribution plate so that, as the comminuted materials come into contact with the distribution plate, the comminuted materials are driven by the spinning distribution plate by centrifugal force outwardly towards the upright wall.
Preferably, the distribution plate is provided with a plurality of acceleration ridges on a top surface of the distribution plate.
The rotational distribution mechanism may further include a mechanism for raising and lowering the distribution plate relative to the upright wall.
The feed means may include a plurality of feed mechanisms with each feed mechanism including a hopper, an auger tube connected to the hopper, an auger shaft disposed within said auger tube an auger screw provided at one end of said auger shaft, a plurality of paddles provided at a second end of said auger shaft, an auger drive motor for driving said auger shaft, and a delivery chute connected to said auger tube adjacent to said plurality of paddles. This arrangement allows a controlled feed of the dry comminuted materials into the blending enclosure.
In another embodiment, the dry blending apparatus may include a vibration mechanism for vibrating the blending container or a part thereof.
In yet another embodiment, the dry blending apparatus may include a rotation mechanism for rotating the substantially upright wall. This rotation may be in the same direction as, or in a direction opposite to, the direction of rotation of the rotational distribution mechanism.
According to another embodiment of the invention, a dry blending apparatus for blending dry comminuted materials includes a vertical cylindrical wall defining a blending enclosure, a feed mechanism for feeding dry comminuted materials into said blending enclosure, a distribution shaft positioned on a centre-line of said blending enclosure, a distribution motor for rotating said distribution shaft, a distribution plate attached to said distribution shaft and positioned such that said dry comminuted materials from said feed mechanism contact said distribution plate whereby said dry comminuted materials are distributed against said wall by centrifugal force, and a tapering chute in communication with said blending enclosure for collecting blended dry comminuted materials as the blended dry comminuted materials slough off the cylindrical wall due to the force of gravity.
According to yet another embodiment of the invention, there is provided a method of blending dry comminuted materials comprising the steps of distributing the dry comminuted materials against a substantially upright wall by centrifugal force and allowing blended dry comminuted materials to slough off the wall due to gravity. This method may also include a step of feeding the dry comminuted materials into a blending enclosure defined by the substantially upright wall prior to the distributing step.
The various features of novelty which characterize the invention are pointed out with more particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be made to the accompanying drawings and descriptive matter in which there are illustrated and described preferred embodiments of the invention.